The invention relates to an X-ray tube which comprises an at least partly metal housing and an electrode which can be connected to a positive high voltage with respect thereto and which is mounted on a ceramic insulator, which projects into the tube, a connection area being surrounded by a screening sleeve which can be connected to the electrode potential.
An X-ray tube of this kind is known from GB 1,272,498; therein, the housing is made of metal and is connected to an anode via an insulator (in the form of a truncated cone.)
It is a drawback of the known X-ray tube that electrons can be emitted from the metal housing by field emmision; such electrons reach the anode along the insulator surface. After having travelled a given distance, such an electron has gathered enough energy to release other electrons which themselves release electrons again etc., so that across the insulator surface an electron avalanche occurs which causes substantial disturbances and development of gas in given circumstances or even a breakdown of the insulator.
This drawback is avoided in an X-ray tube disclosed in DE-OS 25 06 841, corresponding to U.S. Pat. No. 4,053,802. The anode and the metal housing thereof are interconnected by the way of an insulator which comprises a hollow space in the form of a truncated cone which becomes larger towards the cathode. In such an insulator configuration, an electron encounters an electric field across substantially the entire insulator surface which accelerates the electron directly from the insulator to the anode, that is to say via the vacuum space, so that discharges on the insulator surface are prevented to a high degree.
It is a drawback of this known X-ray tube, however, that due to the high relative dielectric constant of the ceramic insulator (approximately 10) the electric field is concentrated mainly in the space between the anode and the surface of the insulator which faces the anode. Consequently, at the area where the anode is connected to the ceramic insulator very high electric field strengths occur on the insulator surface which may cause breakdowns and other faults.
The same problem is encountered in rotary-anode X-ray tubes such as described in DE-PS 24 55 974, corresponding to U.S. Pat. No. 4,024,424 in which a shaft which supports the anode disk is rigidly connected to a ceramic insulator which itself is connected to a rotor.